Prepare the Dressing : In a small bowl, whisk together mayonnaise, Greek yogurt, lemon juice, honey, salt, and pepper until smooth and well combined.

Prepare the Salad Ingredients : Dice cooked chicken, apple, and halve grapes. Slice celery and finely chop red onion if using. Toast walnuts in a dry skillet over medium heat until lightly browned and fragrant.

Combine the Salad : In a large bowl, combine chicken, apples, grapes, celery, and red onion. Add the dressing and toss gently to coat all ingredients evenly. Fold in toasted walnuts last to maintain crunch.

Assemble the Salad : Place mixed greens on a serving platter or individual plates. Spoon the chicken salad on top and garnish with extra walnuts or a few grape halves.